11:45 AM - 12:00 PM
[SMP23-08] The stabilities of SiC polytypes at high pressure and temperature
Keywords:silicon carbide, high temperature and pressure, Earth's deep interior, crystal structure
Although silicon carbides (SiC) are rarely found in geological samples, they potentially play an important role in understanding the global volatile cycle and the redox state of the deep Earth. Enigmatic SiC found in nature is suggested to be created by the hydrogen-rich fluid arising from the subducted materials in the Earth's deep interior. On the other hand, recent studies have indicated that SiC may be a major constituent material of carbon-rich exoplanets. Therefore, the stabilities of SiC polytypes have been widely studied to understand the structure and dynamics of the planetary interior. However, previous studies are inconsistent with each other in the stable polytypes of the SiC at high temperature and pressure conditions, likely due to the strong kinetics of the transition. Here, we have studied the stable phase relations of the 6H-SiC and 3C-SiC under deep Earth and planetary conditions by high-pressure experiments.
We used a Kawai-type multi-anvil apparatus installed at the Institute for Planetary Materials Research to generate high pressure and temperature. A LaCrO3 heater was used to generate the high temperature. The temperature was measured by a W3%Re-W25%Re thermocouple. The starting materials were 3C-SiC and 6H-SiC fine powder standards distributed by the AIST. The experiments were conducted at 20 GPa and 2000 - 2500 K, and the heating durations were 1 hour, which is significantly longer than the previous annealing experiments. After the experiment, the microscopic texture and chemical compositions of recovered samples were studied by scanning electron microscope (SEM) and an energy-dispersive X-ray spectrometer (EDS). The crystal structures of the samples were investigated by powder X-ray diffraction measurements (XRD) and micro-focused laser Raman spectroscopy. The obtained powder XRD patterns were analyzed using Rietveld analysis to quantify the abundance of the phases. We also tried single-crystal XRD measurements to determine the crystal structure of the recovered sample.
The results of SEM-EDS analysis indicated that the samples did not decompose into Si and C. We observed no silicon-carbide compound other than SiC. The XRD measurements showed that the 3C-SiC keeps the original crystal structure. On the contrary, 6H-SiC showed a significant change in peak intensity ratio compared to the starting materials. This result indicates a partial phase transition of 6H-SiC to 3C-SiC. XRD measurements showed that about half of the 6H-SiC has transformed to 3C-SiC under the current experimental conditions. Our results suggested that 3C-SiC is more stable than 6H-SiC at 20 GPa and 2000-2500 K. Although we did not observe the complete transition, we can accurately determine the reaction rate by stable and homogeneous heating in the multi-anvil apparatus. Based on the experimental results, we will discuss the stable crystal structure of SiC in deep planetary conditions.
We used a Kawai-type multi-anvil apparatus installed at the Institute for Planetary Materials Research to generate high pressure and temperature. A LaCrO3 heater was used to generate the high temperature. The temperature was measured by a W3%Re-W25%Re thermocouple. The starting materials were 3C-SiC and 6H-SiC fine powder standards distributed by the AIST. The experiments were conducted at 20 GPa and 2000 - 2500 K, and the heating durations were 1 hour, which is significantly longer than the previous annealing experiments. After the experiment, the microscopic texture and chemical compositions of recovered samples were studied by scanning electron microscope (SEM) and an energy-dispersive X-ray spectrometer (EDS). The crystal structures of the samples were investigated by powder X-ray diffraction measurements (XRD) and micro-focused laser Raman spectroscopy. The obtained powder XRD patterns were analyzed using Rietveld analysis to quantify the abundance of the phases. We also tried single-crystal XRD measurements to determine the crystal structure of the recovered sample.
The results of SEM-EDS analysis indicated that the samples did not decompose into Si and C. We observed no silicon-carbide compound other than SiC. The XRD measurements showed that the 3C-SiC keeps the original crystal structure. On the contrary, 6H-SiC showed a significant change in peak intensity ratio compared to the starting materials. This result indicates a partial phase transition of 6H-SiC to 3C-SiC. XRD measurements showed that about half of the 6H-SiC has transformed to 3C-SiC under the current experimental conditions. Our results suggested that 3C-SiC is more stable than 6H-SiC at 20 GPa and 2000-2500 K. Although we did not observe the complete transition, we can accurately determine the reaction rate by stable and homogeneous heating in the multi-anvil apparatus. Based on the experimental results, we will discuss the stable crystal structure of SiC in deep planetary conditions.